Electrography, which broadly includes the forming and developing of electrostatic image patterns either with or without light, has become a major field of technology. It perhaps is best known through the use of electrophotographic office copying machines. In electrophotographic processes, a uniform electrostatic charge is placed on a photoconductive insulating layer. The layer is then exposed to a light and shadow image to dissipate the charge on the areas of the layer exposed to light. The resulting electrostatic image is developed by depositing a toner powder on the image. The toner powder is only adherently attracted to those areas of the layer which retain a charge so that the toner image corresponds to the electrostatic image when the charging polarity is opposite that of the toner polarity. Conversely, if the toner polarity is the same as the charging polarity, exposed and thus discharged areas of the image can be toned if the potential applied to the toning assembly is higher than that of the exposed areas. The toner image is then transferred to a receiver sheet typically consisting of a smooth, high quality paper such as clay coated lithographic paper stock to which it is permanently fixed thereto by thermal fusion.
Fixing of the toner image to the receiver sheet usually is accomplished by passing the sheet, on which the toner particles are deposited, through the nip of a pair of heated fusing rolls. The roll which contacts the toner usually has a resilient surface such as silicone rubber which has low adhesion to the fused toner. A desirable quality of the thermoplastic toner particles is that they include a toner binder (i.e., a polymer) that has a relatively low fusing temperature, e.g., less than about 250.degree. C. and preferably from about 100.degree. to 250.degree. C. If this fusing temperature is too high, the energy requirement for the fusion step is excessive and the machine life can be reduced by the degradation effects of heat on elastomeric fusing roller materials, electronic components, and the like, and if too low the toner particles tend not to adhere to the receiver sheet. Another desirable quality of the thermoplastic toner particles is that they include a polymeric toner binder that displays a low melt viscosity e.g., in the range of from about 4.times.10.sup.2 to 5.times.10.sup.4 poise as measured on a Rheometrics Dynamic Analyzer at 150.degree. C. and a frequency of 1 rad/sec. Such low melt viscosity is needed to achieve the desired fusing properties such as good surface gloss and the elimination of light scattering voids within an image, good adhesion of the toner to the sheet, good image clarity and high fusing speeds while at the same time allowing for low enough input energy or temperature such that a high quality paper receiver such as a clay coated lithographic paper stock does not blister, char or burn. Blistering is a phenomena where water within the clay coated lithographic paper stock is vaporized during the toner fusing process, causing the paper to form surface protrusions and delaminations. Still another desirable quality of the thermoplastic toner particles is that they include a polymeric toner binder that minimizes "off-setting" of individual toner particles of the developed image during the fixing operation.
Off-setting is the undesirable transfer of toner particles from the developed toner image carried on a receiving member (e.g., copy sheet) to the surface of the heated fusing member (e.g., a fuser roller). The surface of the fusing member therefore becomes contaminated with toner particles; and, upon further use of such a contaminated fusing member, it is found that these toner particles adhered to the surface of the fusing member are transferred to subsequent copy sheets or receiving members. As a result, either a ghost image of previously fixed images is formed on subsequent copy sheets, or undesirable deposits of toner material are formed in background areas of subsequent copy sheets, causing scumming or discoloration in the background areas. In addition, in some instances the copy sheet may fail to separate from the heated fusing member and, in the case of a fuser roller, for example, wrap itself around the roller. Thus, a high "hot offset" temperature, i.e., the temperature at which the cohesive strength of the toner matrix material (or binder resin) is lost and the toner thus sticks to the fusing member and causes offset, also is desirable for a toner. The difference between the "onset of fusing" temperature and the "hot offset" temperature is referred to herein as "offset latitude". The greater the offset latitude is, the wider the temperature range in which the fusing roller can operate. Resistance to offset normally is associated with high melt cohesive strength or high melt elasticity of the polymeric toner binder. Typically, this should range from about 1.5.times.10.sup.2 to about 4.times.10.sup.4 dynes/cm.sup.2, preferably from about 5.times.10.sup.2 .to about 4.times.10.sup.4 dynes/cm.sup.2.
A problem with many polymers which would otherwise be useful in toner compositions is that those with low enough fusing temperatures and a sufficiently low melt viscosity for good flow and adhesion to the receiver sheet, also have a low melt elasticity. As a result, portions of the toner offset onto the resilient fusing roller when the receiver sheet passes through the heated nip. The melt elasticity and, thus, the cohesiveness of the molten toner mass is so low that the fused toner mass undergoes melt fracture when the receiver sheet leaves the nip and separates from the fusing roll. Therefore, although most of the toner sticks to the receiver sheet, some of it sticks to the roller and then offsets onto the next receiver sheet passing through the nip thereby creating an offset or ghost image on that sheet. In addition to the aforementioned off-setting problem, toner binders having a low melt elasticity also exhibit narrower offset latitudes and poor keeping properties. Further, they also exhibit increased brittleness which causes the toner particles to become excessively finely divided during use in the electrostatographic copying machine where they contaminate the inside of the machine and cause a reduction in developer life.
To increase melt elasticity, a number of approaches have been taken. One approach is to crosslink the thermoplastic binder resin of the toner as disclosed in the patent to Jadwin, U.S. Pat. No. Re. 31,072. Crosslinking does reduce toner offset by increasing the cohesiveness of the melted toner but at the expense of raising the fusing temperature and melt viscosity.
Since few, if any, individual fusible polymers have the combination of desired qualities of low fusing temperature, low melt viscosity and high melt elasticity, attempts have been made to form suitable toner compositions from blends of polymers. It is usually found, however, that when a toner binder or polymer of sufficiently low fusing temperature and low melt viscosity is blended with another high molecular weight polymer to form a blend having a satisfactorily high melt elasticity, the melt viscosity of the blend is too high for satisfactory use as a toner.
Thus, further improved toner compositions are needed which at relatively low temperatures will have sufficiently low melt viscosities to flow and fix to receiver sheets, but which will have sufficiently high melt elasticities so that the toner, as it adheres to the receiver sheet, will pull away from the fusing roller and not stick to it.